Muscles stretch and contract when we walk, and skin splits open and knits back together when we get a paper cut. To study these contractile forces, researchers built a three-dimensional scaffold that mimics tissue in an organism. First, they poured a mixture of cells and elastic collagen over microscopic posts in a dish. Then they studied how the cells pulled and released the posts as they formed a web of tissue. To measure forces between posts, the researchers developed a computer model. Their findings—which show that contractile forces vary throughout the tissue—could have a wide range of medical applications. Courtesy of Christopher Chen, University of Pennsylvania.